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Young cancer fighter Jayden Halliwell got only a cardboard mock-up of the green button he will push when he blows up his first building - but he pressed it several times with gusto and a hint of smile.

Jayden, 6, has been given the task of imploding the 14-storey Radio Network House building in Christchurch on Sunday after eight demolition companies pooled together their money to win a Trade Me auction. Their bid was $26,000.

Jayden, from Queenstown, has kidney cancer and is in Christchurch undergoing his first round of chemotherapy with surgery to follow, his mother Justine Halliwell said.

"He is quite a shy boy so this is really overwhelming for him - we really weren't expecting all this," she said today.

"All she said was that they had something very special for Jayden to do on Sunday," said Ms Halliwell.

Ms Furley said Jayden was chosen because he is a "very special boy" and he was also the only child from the cancer ward who was well enough to do the honours.

The eight representatives from each demolition company met Jayden at a press conference today and stood protectively around his wheelchair and his mother as the weather and media closed in.

Jim Shilton of Shilton and Brown Demolition said "As an industry we are pleased we can give something back to the restoration of Christchurch's heritage buildings. Being able to offer someone like Jayden a once-in-a lifetime opportunity is an added bonus."